Superluminescent light emitting diodes (SLEDs) are diodes that, when biased in the forward direction, become optically active and generate amplified spontaneous emission over a wide range of wavelengths. In contrast to laser diodes, there is not sufficient feedback to achieve lasing action (“lasing” here is used to describe the function principle of a laser, i.e. to generate, by a feedback, stimulated emission in a gain medium pumped to provide population inversion and placed in a cavity providing the feedback, resulting in coherent radiation). This is usually achieved by the joint action of a tilted waveguide in which the generated radiation is guided and anti-reflection coated end facets. A tilted waveguide in this context is a waveguide which is not perpendicular to a plane defined by end facets of the device.
SLEDs (sometimes also called Superluminescent diodes, SLDs) are attractive for applications in which a higher intensity than the one emitted by conventional LEDs is required, but where an even distribution of the emitted wavelength over a broad spectral range is desired. In a SLED for delivering a large incoherent light output from the first end facet, it is thus important to suppress laser oscillation.
U.S. Pat. No. 5,252,839 discloses a way to, in addition to non-perpendicular end facets and AR coatings, further reduce feedback generation and thus the tendency of the device to lase. This is done by providing two regions in a waveguide structure with a PN-junction, the first region being forward biased to serve as gain region, and the second region being reverse biased to serve as optical absorber region by Stark absorption or Franz-Keldysch effect. In this way, power loss in the cavity is further enhanced, feedback further reduced, and a brighter non-lasing light source becomes possible, at the cost of a more complicated set-up.
Modern SLEDs include devices with conventional bulk structures as well as devices with structures composed of a plurality of very thin layers (so-called quantum well structures) deposed on a single crystal substrate. Mass production of such bulk semiconductors and quantum well structures is well developed and would allow the production of SLEDs rationally and economically. However, the tilted waveguide or other means of achieving non-perpendicular end facets make it difficult to package SLED devices by standard production methods.